1. Field of the Invention
The present invention relates to a molecular sieve and more particularly, to a carbon molecular sieve. The present invention also relates to a fuel cell and more particularly, to a catalyst and a catalyst support for a fuel cell.
2. Description of the Related Art
Originally, the term “molecular sieve” was the trade name of a synthetic zeolite commercially manufactured by Linde Ltd. (USA). A molecular sieve has excellent adsorption capacity due to its many fine pores with uniform diameter. As derived from its literal meaning, a molecular sieve is used to selectively sieve molecules.
Presently, the term “molecular sieve” is an generic name in the field of science technology and refers to a porous material in which uniform-sized pores are arranged in a three dimensional manner.
A molecular sieve selectively adsorbs a certain size of molecules due to its uniform pore size. Therefore, a molecular sieve can be widely used as a catalyst, a catalyst support, or an adsorbent.
A carbon molecular sieve is mainly made of a carbon material. A carbon molecular sieve has many advantages, such as excellent thermal stability, hydrothermal stability, chemical resistance, and lipophilicity, when compared to a metal oxide based molecular sieve such as zeolite. A carbon molecular sieve can also be used for various purposes, such as a catalyst support, an adsorbent, a sensor, and an electrode material.
As one example of a method for synthesis of a carbon molecular sieve, there is disclosed a pyrolysis process of a natural vegetable material, such as coconut, or a synthetic polymer. According to this method, however, pore size and porosity can only be limitedly increased by pyrolysis temperature adjustment and post-treatment with oxygen.
Another example of a method for synthesis of a carbon molecular sieve is disclosed in Korean Patent Application Laid-Open Publication Nos. 2001-1127 and 2002-84372. According to these methods, a mesoporous silica molecular sieve is used as a template. A carbohydrate is subjected to adsorption into the template, polymerization and pyrolysis. The template is then removed to thereby produce a carbon molecular sieve with a structural regularity of uniform-sized pores. However, a disadvantage exists in that a silica molecular sieve used as a template must be newly designed to adjust the surface area of the carbon molecular sieve and the volume ratio of its micropores and mesopores. In addition, it is difficult to control the volume ratio between micropores and mesopores.
Fuel cells are clean energy sources capable of reducing dependence on fossil energy, with a high output density and high energy conversion efficiency. In addition, fuel cells can be operated at room temperature and can be miniaturized and packed. Therefore, fuel cells can be widely used in the fields of zero emission vehicles, domestic power systems, mobile communication equipment, medical instruments, military equipment, aerospace equipment, and portable electronic devices. A polymer electrolyte membrane fuel cell (PEMFC) and a direct methanol fuel cell (DMFC) are electric power generating systems that allow electrochemical reaction of hydrogen or methanol, water, and oxygen to produce direct current electricity. These fuel cells comprise an anode and a cathode which receive a liquid/gaseous reactant and a proton conducting membrane interposed between the two electrodes. At the anode, an anode catalyst dissociates hydrogen or methanol to generate protons. The generated protons are transported through the proton conducting membrane to the cathode. At the cathode, the protons react with oxygen by the cathode catalyst. Therefore, in such structured fuel cells, the role of a catalyst is very important. Currently, in a PEMFC, platinum (Pt) particles supported on a carbon support are used as both anode and cathode catalysts. In DMFCs, platinum-ruthenium (Pt—Ru) black is used as an anode catalyst and Pt particles by themselves or Pt particles supported on a carbon support are used as a cathode catalyst. Because metal black by themselves provide excellent catalytic activity, a supported metal catalyst system is rarely used in a DMFC. However, because a large portion of costs incurred in a DMFC is caused by a catalyst, in considering cost effectiveness, the amount of a used catalyst needs to be decreased. Therefore, many efforts have been made to research a carbon support capable of providing improved catalyst activity and dispersion over a currently used carbon support with no structural regularity, in order to reduce the catalyst amount used in an anode and a cathode.